1. Field of the Invention
The invention relates to a magnetic resonance imaging apparatus which includes a gradient coil system for generating a magnetic gradient field in an imaging volume of the apparatus, the gradient coil system including a first coil for generating a first part of the gradient field, and a second coil for generating a second part of the gradient field.
2. Description of Related Art
A magnetic resonance imaging apparatus of this kind is known from U.S. Pat. No. 5,311,135.
A magnetic resonance imaging apparatus for medical purposes, also referred to as an MRI apparatus, is arranged to form images of cross-sections of a body. To this end, in such an apparatus a strong, homogeneous field is generated in a volume intended for imaging (the imaging volume) in known manner. On this homogeneous field a gradient field is superposed in order to indicate the location of the cross-section to be imaged. The gradient field is realized by conducting a time-dependent current through a gradient coil. This time-dependent current signal varies in a pulse-like manner, the pulse being shaped approximately as a trapezium. The rise times are of the order of magnitude of from 0.2 to 0.6 ms and the pulse duration (i.e. the duration of the more or less constant part of the pulse) is of the order of magnitude of from 1 to 5 ms.
Depending on the intended application and/or the user's wishes, the design of a gradient coil system may be aimed at achieving a more or less high degree of linearity of the gradient field or a more or less high speed in generating the gradient pulses.
In this context linearity is to be understood to mean the degree of constancy across the imaging volume of the (spatial) derivative of the field strength of the gradient field (for example, the derivative of the z component of the gradient field to the x co-ordinate: dB.sub.z /dx). For non-distorted images a high degree of linearity is pursued in the imaging volume. For reasons of power efficiency, during the generating of a very linear field the field outside the imaging volume is preferably decreased to zero as quickly as possible (i.e. over an as short as possible distance from the imaging volume). This is because the power required for generating a magnetic field is proportional to the volume integral of the square of the field strength. These two requirements (linearity within the imaging volume and fast field decrease outside the imaging volume) must be satisfied in the practical circumstances of an MRI apparatus, i.e. the imaging volume may not be completely enclosed by current conductors, because this volume must remain accessible by a patient, and the current conductors may not be arranged directly on the boundary surface of the imaging volume. Considering the electromagnetic field theorem, in these circumstances said requirements as regards linearity and fast field decrease are not compatible, so that a comparatively high power is required in order to generate a very linear field.
There are also MRI applications in which significant importance is attached to a high speed and linearity is less important. Such applications are encountered in situations in which images must be formed at a high speed, for example in the case of moving parts of the body. Examples in this respect are the observation of a beating heart or the tracking of the progress of a contrast medium in a vascular system as it occurs during perfusion imaging of the brain. As is known, in MRI the resolution of the image to be formed (i.e. the number of pixels that can still be distinguished from one another in the slice to be imaged) is proportional to the surface area of the gradient pulse. For example, if the speed is made twice as high while the resolution remains the same, so for the same surface area of the gradient pulse, the amplitude of the pulse will have to be twice as high; this means that the slope of the pulse edges of the trapezium-shaped pulses should become four times as high, so that the driving voltage V for this coil (behaving practically as a pure self-inductance L, i.e. V=L(dI/dt)) should also become four times as high. Because the current amplitude I has thus become twice as high, the peak power V.times.I becomes eight times as high. This numerical example clearly illustrates the problem concerning the driving power required in the case of a high speed.
Therefore, if a gradient system were desired which is capable of producing a high degree of linearity as well as a high speed, driving amplifiers designed to deliver a very high power would be required; this is objectionable. Moreover, a high driving power is accompanied by a high heat dissipation, giving rise to cooling problems and mechanical instability due to thermal drift.
The MRI apparatus described in the cited United States patent includes a gradient coil system which consists of a series connection of a first and a second coil, either the first coil alone or the first as well as the second coil being excited. Such a method of operating the coil system enables an increased speed of acquisition of the images in the case of a part of a patient to be imaged which is smaller than the imaging volume. A situation of this kind occurs, for example when images are formed of the human brain which has dimensions of the order of magnitude of 20 cm, whereas the diameter of the imaging volume is approximately 45 cm. The gradient field will be more linear across said 20 cm than across the diameter of 45 cm, so that the requirement in respect of linearity across the entire imaging volume may be less severe in this case. The reduction of the linearity may then be of benefit to an increased speed. If only the first coil is excited, a fast magnetic field having a given linearity can be realized in a given imaging volume. When the first as well as the second coil is energized, the volume having said given linearity will be larger than the imaging volume in the first case or, in other words, when both coils are excited, the linearity within the former volume, so the imaging volume, will be higher.
Thus, in the known MRI apparatus a choice can be made between a first gradient mode in which only the first coil is activated, so that only this first coil is excited whereas the second coil is not, and a second gradient mode in which the first and the second coil are activated together, so that the same current flows through the two coils. Selection between said two gradient modes thus enables the user to choose either a comparatively high linearity with a comparatively low speed or a high speed with a low linearity.
The second coil shown in the cited United States patent is proportioned in such a manner that it produces a field whose linear component constitutes a significant part of the linear component of the total field produced by the gradient system. This means that when the second coil is deactivated, the total linear field component is reduced by an amount which is not negligibly small, so that in that case the excitation of the first coil must be increased by a corresponding amount so as to achieve the same strength of the total linear field component; this has an adverse effect on the power efficiency during the generating of the linear field.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant's invention of the invention subsequently claimed.